Why storage matters
Supply and demand rarely line up. Rain comes in storms. A well pump runs a few minutes at a time. You use water every day.
Storage bridges the gap. A tank holds water from times of plenty for times of need. On a rain-fed homestead, the tank decides how long you can go without rain.
Set a tank size, people and daily use. How long does a full tank last with no rain?
About 31 days, or 4.5 weeks.
Liters: 9,464 L in the tank.
Assumptions: Ignores garden water and the bottom few inches of a tank you should not draw from.
Tank materials
Tanks come in several materials. All must be opaque, to stop algae, and sealed against insects, animals and sunlight. If the water might be treated for drinking, the tank and fittings should be rated for potable water.
Water is heavy
One gallon of water weighs about 8.3 lb (1 liter weighs 1 kg). A 1,000-gallon tank holds about 8,300 lb, more than 4 tons.
That weight must sit on a firm, level base. A soft or sloped base can let a tank crack, tip or burst. Raising a full tank onto a stand or tower is a structural job. A failed stand can hurt someone badly.
Place tanks close to the downspouts and the house to keep pipe runs short. Keep them away from septic systems, and where a truck can reach if you ever need water delivered.
Gravity makes pressure
Water pressure is measured in pounds per square inch (psi). Height creates pressure on its own. Every foot of height between the water surface and the tap adds about 0.43 psi. Put another way, it takes about 2.3 feet of height for each psi.
A tank 20 feet (6 m) above a tap gives about 9 psi. That is enough for a garden hose or a slow tap. Most household fixtures are designed for much more. Many homes run at roughly 40 to 60 psi. EPA’s WaterSense program suggests 45 to 60 psi for the water coming into a home.
To get 40 psi by gravity alone, a tank would need to sit about 90 feet above the house. So most homesteads use a pump, or gravity only for a garden.
Text description
Left, a sketch of a water tank on a hill above a house, with the height measured from the water surface in the tank down to the tap. Right, a chart of pressure against that height. The line rises about 0.43 psi for each foot, or about 2.3 feet for each psi. A tank 20 feet (6 meters) above the tap gives about 9 psi, enough for a garden hose or a slow tap. Getting about 40 psi takes about 90 feet (27 meters) of height. A shaded band from about 40 to 60 psi marks where many homes run.
Pumps and pressure tanks
A pump pushes water from the tank or well to the house. Many systems pair it with a pressure tank: a small tank with an air cushion inside. The air keeps the water under pressure, so the pump does not start every time you open a tap.
Text description
A block diagram. Water flows from a storage tank or well to a pump. The pump needs electricity, and choosing and wiring it is work for a pump contractor and a licensed electrician. From the pump, water flows into a pressure tank, which has an air cushion above the water. The squeezed air keeps the water under pressure. From the pressure tank, water flows to the taps in the house. The pump runs now and then to refill the pressure tank, rather than every time a tap opens.
Pumps need electricity. On an off-grid site, the pump can be one of the biggest loads (lesson 22). A pump that cycles on and off too often wastes energy and wears out early.
Choosing and wiring a pump is trade work. Pump size depends on how high the water must rise, how far it travels, and the flow you need. A pump contractor works this out.
Freeze protection
Water expands when it freezes. Ice can split pipes, fittings, pumps and tanks. In cold climate zones, freezing is a design problem, not an afterthought.
Common approaches include:
- Burying pipes below the local frost line, the depth the ground freezes in a hard winter. It ranges from almost nothing in the far South to about 5 feet (1.5 m) in the coldest parts of the lower 48 states.
- Burying the tank, or putting it inside a heated or insulated space.
- Draining above-ground lines and barrels before winter.
- Heat cable on exposed pipes. This is an electrical load and must be installed to code.
Text description
Left, a section through the ground at one site, not to scale. The top layer of soil freezes in a hard winter; its lower edge is the frost line, shown as a dashed line. A water pipe inside the frozen layer can freeze and split. A water pipe buried below the frost line stays unfrozen. A note says your local building department knows your frost depth. Right, a comparison of frost depth across the lower 48 states: almost nothing in the far South, and about 5 feet (1.5 meters) in the coldest parts.
Your local building department knows your frost depth.
Diagnose it
“The rain tank ran dry in August.”
Clues
- The 1,500-gallon tank was sized from the yearly rainfall total.
What is the most likely cause?
Pro and permit
Check
A tank sits on a hill 46 feet above the house. About how much pressure does gravity give at the tap?
Summary
- Storage bridges the gap between when water arrives and when you use it.
- Tanks must be opaque and sealed. Water is heavy: about 8.3 lb per gallon.
- Every foot of height gives about 0.43 psi. Most houses need a pump and a pressure tank.
- Size tanks for the dry season. In cold zones, plan for freezing from the start.
Go deeper: why the bottom of a tank is not usable
Fine sediment settles at the bottom of a tank. Most systems draw water a few inches above the floor, often with a floating intake that sips from just below the surface where water is cleanest. That means a tank’s usable volume is always a bit less than its label. Tanks also need a sealed access hatch, a screened vent and a screened overflow, so air can move without letting insects or animals in.